Conventional neutron tubes employ a Penning ion source and a single gap extractor. The target is a deuterium or tritium chemical embedded in a molybdenum or tungsten substrate. Neutron yield is limited by the ion source performance and beam size. The production of neutrons is limited by the beam current and power deposition on the target. In the conventional neutron tube, the extraction aperture and the target are limited to small areas, and so is the neutron output flux.
Commercial neutron tubes have used the impact of deuterium on tritium (D-T) for neutron production. The D-T reaction produces 14.1 MeV neutrons. The deuterium-on-deuterium (D-D) reaction, with a cross section for production a hundred times lower, has not been able to provide sufficient neutron flux. The D-D reaction produces 2.45 MeV neutrons.
Neutronics can identify possible explosives and nuclear materials by their composition, not just by their shape or density the way x-ray machines do. Since the September 11 terrorist attacks, detection of explosives and fissionable materials has become an urgent national need. Detecting such materials hidden in baggage or cargo is challenging under real-world conditions.
Thermal neutron analysis (TNA) has been tried for inspection of checked baggage and cargo at airports. Low-energy neutrons cause nitrogen in explosives to emit gamma rays and cause fissile materials to give off neutrons of their own. The first-generation TNA screeners were too large, complex, and expensive; furthermore, TNA requires moderators to slow fast neutrons from a source to thermal neutrons. FAA-approved screening devices presently on the market use x-rays to look at shapes and densities, rather than using neutronics to detect actual composition.
Fast neutron interrogation of luggage and cargo for contraband such as explosives and narcotics has also been proposed. In particular, neutron absorption spectroscopy is a technique in which materials are identified by absorption of neutrons from a neutron source and emission of gamma rays which are detected with gamma ray detectors. To identify different materials, a broad neutron energy spectrum, typically 1 MeV to 9 MeV is required, since each element absorbs at a different neutron energy.
Thus a neutron source with a broad energy spectrum over the 1 MeV-9 MeV range is desired for a baggage or cargo interrogation system based on neutron absorption spectroscopy. A D-D source is unsuitable because it is too low energy. While a D-T source may be used, it produces too high energy neutrons which must be slowed by moderators to the desired energy range, resulting is larger sized systems with large loss of neutron flux and loss of beam directionality.
An interrogation system based on the D-Be reaction has been proposed. D+ ions from an ion source injector are accelerated in a cyclotron to 9 MeV and then directed onto a Be target to produce neutrons. The system is low current, large and complex.
Besides the obvious considerations of cost-effectiveness and acceptable footprint, systems for inspecting baggage and cargo must offer trustworthiness (reliability with freedom from both false positives and false negatives), plus high throughput so that spot checks can be replaced by comprehensive inspection without bottlenecking an already heavily burdened process. Fast interrogation of about 2 sec/piece of luggage is preferred. Systems are also needed for relatively nonintrusive inspection of larger objects, e.g. an intermodal cargo container, or a vehicle.
Plasma fusion researchers have studied the plasmas of hydrogen isotopes. In addition to the D-D and D-T reactions described above, the T-T reaction also produces neutrons, by the reaction T+T→4He+2n. The reaction produces two neutrons per reaction compared to one neutron for each D-D or D-T reaction, and the neutrons have a continuum energy spectrum extending up to 9.4 MeV. However, this reaction has not been used in conventional neutron sources.
Therefore, in accordance with the invention, a compact neutron generator based on the T-T reaction, and a luggage and cargo interrogation system based on a T-T neutron generator, would be highly advantageous.